The retinoblastoma (Rb) protein is a normal cellular constituent that is intimately involved in the regulation of the cell cycle. The activity of the RB gene is determined by both its expression, and posttranslational modifications, such as phosphorylation, of its protein product. Using molecular biological techniques, we have been able to study the lens, with its cuboidal anterior epithelium, its dividing and differentiating equatorial region. and its terminally differentiated, elongated, nondividing fiber cells, to determine the physiological effects of this key regulatory gene.